Physics
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufmsa33a1135s&link_type=abstract
American Geophysical Union, Fall Meeting 2004, abstract #SA33A-1135
Physics
2411 Electric Fields (2712), 2415 Equatorial Ionosphere, 2479 Solar Radiation And Cosmic Ray Effects, 2481 Topside Ionosphere
Scientific paper
The quiettime diurnal variations of the vertical ion drifts measured by ROCSAT-1 at 600 km equatorial topside ionosphere have been studied for the seasonal and solar flux effects during the moderate to high solar activity years from 1999 to 2003. The vertical drift velocity at 600 km is somewhat larger in magnitude than what has been observed by ground radars but indicates similar diurnal variation pattern of the vertical drifts in the global equatorial vertical drift model of Scherliess and Fejer [1999]. The drift velocity also indicates little dependence on the solar flux intensity during the solstice seasons and some small dependence during the equinox seasons. The well-known feature of the postsunset enhancement which is observed more frequently during equinoxes than during solstices is also noticed for the vertical drifts at 600 km. In addition, ROCSAT observes an outstanding feature of the presunrise enhancement of the downward drift when the solar flux intensity is lower than 140. This prominent presunrise enhancement has been known to exist in many model simulations but was seldom reported in past experimental observations except from the Atmospheric Explorer E [Fejer et al., 1995] over some longitude regions. The statistical occurrence pattern for the presunrise enhancement indicates that it is enhanced during solstices of high solar activities than during equinoxes, which is opposite to the occurrence pattern of the postsunset enhancement. This opposite seasonal occurrence pattern can be understood from the movements of the sunrise and the sunset terminator in local times during equinoxes and solstices, respectively, in relation to the location of the neutral wind reversal around the solar terminator.
Chao Chia-Chun
Su Shangguo
Yeh Henry
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